Fainga'a has signed a one-year deal for next season's Super Rugby campaign as Wallabies captain Stephen Moore moves in the opposite direction to the Reds.
A 36-cap Wallaby, Fainga'a emerged from Brumbies' Canberra-based academy in 2006 and went on to make 27 appearances before joining the Reds in 2009, where he has amassed almost 100 appearances.
"I've thoroughly enjoyed my time with the Reds but equally am looking forward to returning to where it all began for me over a decade ago," the 29-year-old said.
"My focus right now is this season however I'm very glad to have my future sorted for next year."
Fainga'a has played eight games this season for the Reds, who are second-bottom in the Australasian Group after 11 games.
"With Stephen Moore returning to the Reds next year there was obviously a gap we needed to fill in our squad and I think Saia is a perfect fit," said Brumbies coach Stephen Larkham in a statement.
"His aggression, low tackles and fervour at the breakdown will definitely be of great benefit to us next season."